Substation filter having saturable reactor for selectively furnishing frequency dependent coupling under hook switch control



May 5, 1964 E. BAUMAN 3,13

SATURABLE REACTOR FOR SELECTIVELY FURNISHING FREQUENCY DEPENDENT COUPLING UNDER HOOK SWITCH CONTROL Filed Sept. 9, 1957 I0 I I n I I PRIOR ART 9 I 2/ I2 n I SUBSTATION 2O PHYSICAL AND CARR/ER L TELEPHONE LINE FILTER I3 I g- CHARACTERISTICS OF FILTER I3 WITH SWITCH 2! IS CLOSED IMPEDANCE Fig.2

CHARACTERISTICS OF FILTER I3U'79JI WITH SWITCH 2! IS OPEN FREQUENCY 4 FREQUENCY E22 I 2 I 2 I2 26 SATURABLE L J SuBsTATIoN 2O CORE 22 FILTER 23 N SATURABLE F/ .6 I, 9 ig.5 3 I 3 3 30 If I? E E D. C. CURRENT-b CHARACTERISTICS OFSATURABLE CORE 22 WITH SWITCH 2| CLOSED FREQUENCY CHARACTERISTICS OF SATURABLE CORE ZZAND FILTER 23 WITH SWITCH 2! OPEN INVENTOR. EDWARD E. BAUMAN i scribers.

United States Patent Office 3,132,209 Patented May 5, 1964 This invention is directed to filter switches, and more particularly to filter switches which include variable ,impedance means for selectively coupling telephone substations to two-wire transmission lines carrying carrier frequencies.

In the communications field, it'is frequently more economical to transmit simultaneously a plurality of sep arate signals at different frequencies comprising channels over a single pair of conductors, than to provide a separate pair of conductors for each signal or channel. For example, in many rural areas it is more advantageous from a cost standpoint to create additional signalling channels by transmitting different preselected carrier frequencies over existing physical transmission paths than to add additional physical paths. In such type arrangements, each of the different frequencies (fl, f2, f3, etc.) establishes a separate path or channel, and signal intelligence is transmitted over these carrier channels by modulating such frequencies with the audio frequency intelligence to be transmitted. The' regular audio frequency signals are transmitted for physical substationsalong path. V a

In addition to the regulansubstations connected to physical line, carrier substations may be coupled to the physical line through appropriate. )termination arrangements for the purpose of passing only carrier demodulated audio frequency to the carrier substations. The assigned carrier frequency signal is demodulated by the termination equipment associated with thesubstations and the audio frequency component conveyed by such signal is passed to the substation set.

Additionally, the first section of the physical line may end in a carrier termination which serves a group of carrier subscribers, and a high pass filter connected at such .carriertermination permits certain other carrier frequencies to pass toan extended section of the two-wire physical line (which is not metallically connected to the, first section'), for the of this type are more fully Patent 2,875,366 which copending application to to the assignee of this in- These and other installations shown and described in U.S. issued February 24, 1959, on a Bauman et al., and was assigned vention.

It is apparent that no such terminating arrangement is'required for the physical substations which are connected to the transmission line for the reception of audio frequency signals appearing thereon. However, these physicalsubstations, as well as carrier substations which are connected to lines having carrier, frequencies thereon, do require expensive and extended filter systems between the transmission line and the substation to block the modulated carrier frequency energy from the substation, and to simul- .taneously permit the passage of audio frequency (A.-F.) signals. Moreover, in addition to the expense because of their complexity, such filters offer only a partial solution to the problem, since the known types of filters provide a substantial bridging loss, reduce the frequency response of the audio frequency circuit, and when the substation handset is in the on-hook condition (in which condition the filter is not terminated), the filter itself exhibits characteristics that result in selective frequency short circuits.

the existing physical transmission purpose of serving more distant carrier subaudio frequency signals That is, at a certain frequency or frequenciest he filter presents an effective short circuit impedance, thus result: ing in the delection of quency on the line. are inadequate to effectively accomplish the desired purposes.

' The problems set forth hereinbefore would be greatly simplified if a filter switch could be designed to exhibit a high impedance to both carrier and A.-F. signals when the substation is not conditioned for use on-hook condition) and to exhibit high impedance to the carrier signals and only negligible attenuation to such A.-F. signals when the substation is conditioned for use (off-boo condition), and it is a specific object of this invention to provide switching means operable in this manner.

It is another object of this invention to provide switching means, including variable impedance means and filter means, for exhibiting a quency and audio frequency signals when a substation is not in use (see FIG. 5), and for exhibiting high impedance to the. carrier frequency signals and low attenuation to when the substation is conditioned for use.

. The features of the present invention which are believed to be novel are set forth with particularity in the appended claim. The invention itself, and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which: i

FIGURE 1 is a schematic diagram, partially in block form, of a prior art device;

FIGURE 2 is a'graphical illustration useful in understanding the operation of the device shown in FIGURE 1 in which a frequency short within the audio range is indicated;

FIGURE 3 is a graphical illustration useful in understanding the operation of the devices of both FIGURES 1 and 4; v

FIGURE4 isa schematic diagram of an embodiment of the invention; and

FIGURES 5-arid 6 are graphical illustrations useful in understanding the operation of the embodiment of the' invention shown in FIGURE 4. In FIGURE 1 the illustrated transmission line ductors avmg connectmg terminals 11 and 12, respectively, for coupling the input terminals of subscriber drop thereto. The subscriber equipment may basically comsponsive to removal of wishes to initiate a call, or to receive a call as signaled by the actuation of the ringing circuit (not shown) in substation 20.

When switch 21 isopen, filter 13 is not terminated and so exhibits the undesirable selective frequency short circuit characteristics described hereinbefore FIGURE 2. As filter 13 approaches zero at a frequency of the signal applied to the input side offilter 13 increases. It is apparent, therefore, that signals coupled along transmission line 10 will have this frequency appreciably attenuated by the effective short-circuit impedance of filter 13, thus appreciably reducing whatever intelligence was transmitted at such frequency. The impedance-freany signal transmitted at such fre-' It is therefore .clear that such filters high impedance to carrier fre-* together with further objects quency characteristic exhibited by filter 13'when switch 21 is closed (filter 13 is then terminated) is shown in FIGURE 3 in which only a slight drop in impedance occurs above the audio band.

In accordance with the invention, switching means- 27 and 28 of filter 23 are coupled to terminals 18 and 19 of the telephone substation.

When saturable reactors 22A and 22B are utilized as shown in FIGURE 4 and the cores of these reactors have a low value otfiux density, reactors 22A and 22B present a very high impedance to carrier and A.-,F. signals. .As

of the telephone substation components (not shown) across the output terminals of the filter whenever the saturable reactors pass alternating current signals. Thus the ultilization of the novel switching arrangement comprising variable impedance meansanda simplex filter effects a substantial economy in the construction of filters for such I applications.

The variable impedance means of the inventive structure does not exhibit a short circuit impedance to certain a mechanical switch, transmission line, ,such as is experienced in most known the frequency of such signals increases, the saturable;

reactors present an even higher impedance, as illustrated by curve 29 in FIGURE 5 of the drawing. It is'noted that this desirable effect of the variable impedance is achieved even though the output side of filter 23 is not terminated by the closure of switch 21. Thus, even in the on-hook condition of substation 20, a desirable impedance characteristic is obtained as shown inFIGURE 5, in contradistinction to the selective frequency short circuiting shown in FIGURE 2,' which isobtained with prior art filter arrangements.

7 The cores of the saturable reactors may be constructed of suitable magnetizable material, and

a DC. or very low frequency A.-C. control current may core is increased, the value of the impedance to carrier 7 and A.-F. signals presented-by such saturable reactors is sharply decreased. This relationship is illustrated in FIG- URE 6, where the curve 30 is indicative of the impedance to A.-C. signals presented by satur'able reactors responsive to an increase in the D.-C. magnetizing current, which produces a field in the core of such a reactor.

The saturable reactors 22A and 22B oifer negligible impedance to both carrier and A.-F. signals after the D.-C. supervisory current has reduced the impedance of the reactors. However, filter 23is effective to block any carrier frequencysignals and to V the associated substation 2i) Filter 23 may be a simple low-pass arrangement consisting of only a few elements as shown in FIG. 4, because the output side of filter 23 is terminated by the closure of switch 21 and the connection be coupled around the core to vary the degree of its magnetization. Such control current may be provided, for example, by the pass audio frequency signals to shown and described,

' ations as may fall within transmission line and said frequencies as do prior artarrangements, and thus does not diminish the frequency response of the telephone substation circuit in the manner of the prior art devices. The novel structure of the invention acts substantially as there is no bridging loss to shunt the types of filter devices.

While simultaneously providing the foregoing advantages, the. inventive structure is also effective to permit the use of simple and inexpensive filter strutur'es, which require only a few elements.

While a particular embodiment of the invention has been it is apparent that modifications and alterations may be made therein, and it is intended in the appendedclaim to cover all such modifications and alterthe true spirit and scope of the invention. q

What-is claimed is:

In a telephone system, a transmission line over which signals at various frequencies, including carrier frequency and audio frequency. signals and a DC. component are transmitted, substation means including a switch, coupling means including D. C.I responsive variable impedance means and low pass filter means coupled between said substationmeans, said coupling means having a high impedance to both said carrier and said'audio signals when said switch at the substation is open and no D.C. fiows in said coupling means, and said coupling means having a high impedance to said carrier signals and only a negligible impedance to said audio.

i References Cited in the file of this patent V UNITED STATES PATENTS 2,151,173 Whittle Mar. 21, 1939 2,315,784 Goodale Apr. 6, 1943 2,413,263 Suter Dec. 24, 1946 2,650,350 Heath Aug. 25, 1953 2,694,954 Kock a Nov. 23, 1954 2,708,219 Carver May 10, 1955 2,777,994 Hurault ]an. 15, 1957 Hochgraf Feb. 9, 1960 J as 

